Silicon photonic technology in which optical devices and electronic devices are highly integrated in a hybrid fashion on the same silicon substrate are receiving considerable attention. These techniques are naturally being applied to the field of optical communication, but there are also high expectations for application to the field of optical interconnects in integrated circuits, and such research and development is currently being actively pursued.
In silicon photonic techniques, it is common to use a silicon-on-insulator (SOI) substrate in which a crystal silicon (c-Si) has been formed on a buried oxide film (BOX), but there is a problem in that a process of very high temperatures of 1100° C. or higher is generally required to form c-Si, and SOI substrates are costly.
In view of these problems, silicon photonic techniques have been proposed that use a hydrogenated amorphous silicon (a-Si:H) having optical characteristics that rival c-Si or surpass c-Si in terms of nonlinear optical characteristics in some applications, while being capable of depositing a film at a low temperature of 400° C. or less, and various passive elements have been researched and developed.
However, it is obvious that a high-speed electro-optical conversion device, an optical switching device, or another active device that switches an optical path using an electric signal would be required in order to carry out electrical signal communication using light waves as carrier waves, but there are not many reports related these elements. A possible reason for this is that a-Si:H is an amorphous material, and therefore has poor mobility, electrical conductivity, and other electrical characteristics.
It is known, as indicated in non-patent document 1, that electrons injected or excited inside the a-Si:H by some technique relax in a very short period of time, typically on a sub-picosecond timescale, and this is due to the fact that relaxation of wave function of the carriers from an extended state to a localized state, specifically, a tail state, is extremely fast. Since the tail state originates from fluctuation in the Si bond length or the bonding angle, high-speed carrier relaxation can be said to be a phenomenon unique to amorphous semiconductors, originating from random structures.
Many electro-optical modulators based on c-Si have been reported (e.g., patent document 1), but, in the case of c-Si, one of major factors is that the carrier relaxation time is relatively long and modulation speed is limited. In other words, in terms of the carrier relaxation time, a-si:H can be said to have advantageous characteristics as a high-speed electro-optical modulator in comparison with c-Si.
FIG. 6 shows a cross-sectional schematic view of the conventional electro-optical modulator disclosed in non-patent document 2.
This electro-optical modulator is provided with: an i-type a-Si:H layer 103 as a waveguide core without addition of impurities into a silicon substrate 101; a p-type a-SiC:H layer 102 as a lower cladding layer between the silicon substrate 101 and the i-type a-Si:H layer 103; the p-type a-SiC:H layer being a p-type semiconductor obtained by adding impurities into a hydrogenated amorphous silicon carbide (a-SiC:H) having a slightly lower index of refraction than an i-type a-Si:H while being capable of low-temperature growth in the same manner as an i-type a-Si:H; an n-type a-SiC:H layer 104 to which impurities have been added as an upper cladding layer on the i-type a-Si:H layer 103; a zinc oxide/aluminum electrode 105 disposed thereon.
The electro-optical modulator shown FIG. 6 comprises an optical waveguide structure in which the i-type a-Si:H layer 103 having the highest index of refraction is used as a waveguide core, and at the same time, the p-type layer (102), the i-type layer (103), and the n-type layer (104) constitute a pin structure. In this electro-optical modulator, the electrical conductivity of the a-SiC:H in the n-type layer (104) and the p-type layer (102) is 2.3×10−6 S/cm and 1.9×10−8 S/cm, respectively.
The electro-optical modulator is connected to an external power source so that voltage is applied to the i-type a-Si:H layer 103 by way of the silicon substrate 101 and the zinc oxide/aluminum electrode 105 of the upper part of the waveguide. When a reverse bias is applied to the i-type layer (103), a depletion layer spreads to the p-type layer (102) and n-type layer (104) sides, the carrier density of the i-type layer (103) decreases, and the index of refraction of the i-type a-Si:H layer 103 increases.
Phases of the light waves propagated through a waveguide having the i-type a-Si:H layer 103 as a waveguide core can thereby be modulated. In this case, the operating speed of the electro-optical modulator is limited mainly by the electrical conductivity and mobility of the p-type layer (102) and the n-type layer (104), but in the case of the electro-optical modulator described in non-patent document 2, a-SiC:H, which has very low mobility and electrical conductivity, is used as the p-type layer (102) and the n-type layer (104), and it is therefore very difficult to obtain high-speed modulating operation that exceeds 1 Gbps. In other words, the high-speed carrier relaxation characteristics of a-Si:H are not being taken advantage of in this electro-optical modulator.
In order to solve the above-described problem, the present inventors have proposed a novel electro-optical modulator in an earlier patent application (see patent document 2).
FIG. 7 shows a cross-sectional schematic view of an example of the proposed electro-optical modulator.
This electro-optical modulator is provided with a silicon thermal oxide film 202 obtained by thermally oxidizing a silicon substrate on a silicon substrate 201, and layered thereon in the vertical direction are a boron (B)-added p-type hydrogenated microcrystalline silicon (μc-Si:H) layer 203 having a thickness of about 0.1 μm, a nondoped i-type a-Si:H layer 204 having a thickness of about 1.3 μm, and a phosphorus (P)-added n-type μc-Si:H layer 205 having a thickness of about 1.3 μm.
A silicon oxide film (206), an ITO (indium-tin oxide) film 207, and electrodes (208, 209) comprising aluminum (Al) are furthermore formed thereon.
The layers 203, 204, and 205 each have about the same index of refraction (3.4 to 3.6), have a higher index of refraction than the silicon thermal oxide film 202 having an index of refraction of about 1.44, the silicon oxide film 206, and the ITO film 207, and therefore act as an optical waveguide core and light waves are propagated therethrough.
The optical waveguide constitutes a rib structure having a width of about 3.0 μm, a height of about 1.5 μm, and a rib height of about 0.1 μm. Also, the layers 203, 204, and 205 constitute a pin structure, and electrons or positive holes can be injected into the i-type a-Si:H layer 204.
FIG. 8 shows a cross-sectional schematic view of another example of an electro-optical modulator proposed by the present inventors.
This electro-optical modulator has a p-type semiconductor layer and an n-type semiconductor layer disposed so to be layered in the vertical direction via an optical waveguide comprising an i-type amorphous semiconductor, and electrodes can be drawn out at the upper surface.
The electro-optical modulator shown in FIG. 8 is provided with a silicon thermal oxide film 302 obtained by thermally oxidizing a silicon substrate on the silicon substrate 301, and layered in the vertical direction thereon are a boron (B)-added p-type μc-Si:H layer 303, a nondoped i-type a-Si:H layer 304, and a phosphorus (P)-added n-type μc-Si:H layer 305.
A silicon oxide film 306, an IZO (indium-zinc oxide) film 307, and an electrode 308 are furthermore provided thereon. Also, an IZO film 309 and an electrode 310 are provided as a drawn-out electrode of the boron (B)-added p-type layer 303.
The layers 303, 304, and 305 each have about the same index of refraction (3.4 to 3.6), have a higher index of refraction than the silicon thermal oxide film 302 having an index of refraction of about 1.44, the silicon oxide film 306, and the IZO film 307, and therefore act as an optical waveguide core and light waves are propagated therethrough.
The optical waveguide constitutes a typical wire waveguide structure having a width of about 0.5 μm and a height of about 0.2 μm.
Also, the layers 303, 304, and 305 constitute a pin structure, and electrons or positive holes can be injected into the i-type a-Si:H layer 304.
A portion of the electro-optical modulator proposed by the present inventors was introduced above, and the proposed electro-optical modulator is summarized as follows.
(1) An electro-optical modulator provided with an optical waveguide comprising a silicon-containing i-type amorphous semiconductor, and a silicon-containing p-type semiconductor layer and a silicon-containing n-type semiconductor layer constituting an optical waveguide together with the optical waveguide comprising an i-type amorphous semiconductor and being disposed at a distance from each other via the optical waveguide comprising an i-type amorphous semiconductor, wherein the p-type semiconductor layer and/or the n-type semiconductor layer is a crystalline semiconductor layer.
(2) An electro-optical modulator provided with an SOI substrate, an optical integrated circuit substrate, or another substrate; an optical waveguide comprising a silicon-containing i-type semiconductor formed on a substrate; and a silicon-containing p-type semiconductor layer and a silicon-containing n-type semiconductor layer constituting an optical waveguide together with the optical waveguide comprising an i-type amorphous semiconductor and being disposed at a distance from each other via the optical waveguide comprising an i-type amorphous semiconductor, wherein the p-type semiconductor layer and/or the n-type semiconductor layer is a crystalline semiconductor layer.
The electro-optical modulator is capable of varying the index of refraction of the optical waveguide comprising an i-type amorphous semiconductor by applying voltage or flowing electric current between the p-type semiconductor layer and the n-type semiconductor layer.